This invention relates to a mine roof warning device for signaling the settling of the roof of a mine tunnel or other underground excavation. In underground mining there is constant danger both to the mine workers and the mining machinery, of a shift in the strata above the mine passage tunnel which can eventually result in a cave-in if precautionary measures are not taken. Minor shifts of the strata in the mine ceiling indicative of pending disaster are not always noticable to the naked eye of even the experienced mining engineer.
Previous warning devices for mine roofs have been limited in their application because of their size and expense. All the previous devices known either have required an external power source which added to the fire hazard in the mine or have required periodic inspection by trained personnel.